Lynne A. Werner

Developmental differences in susceptibility to neomycin-induced hair cell death in the lateral line neuromasts of zebrafish (Danio rerio)

Mechanosensory hair cells of the inner ear are susceptible to death when exposed to a variety of drugs including aminoglycoside antibiotics. During avian and mammalian development, there is a period of relative insensitivity to aminoglycoside-induced hair cell death. This study was designed to test the hypothesis that zebrafish (Danio rerio) have developmental differences in sensitivity to aminoglycoside-induced hair cell death in the lateral line neuromasts. Larval zebrafish of various ages were exposed to several concentrations of neomycin, and their hair cells were examined using the potentiometric vital dye, DASPEI. Results indicate that zebrafish larvae aged 4 days post-fertilization are relatively insensitive to aminoglycoside-induced hair cell death compared to older fish. Thus zebrafish hair cells show developmental differences in sensitivity to aminoglycoside-induced death similar to those reported for inner ear hair cells of birds and mammals.

Sources of auditory masking in infants: Distraction effects

Previous work has demonstrated that infants’ thresholds for a pure tone are elevated by a masker more than would be predicted from their critical bandwidths. The present studies explored the nature of this additional masking. In Experiment 1, detection thresholds of 6-month-old infants and of adults for a 1-kHz tone were estimated under three conditions: in quiet, in the presence of a 4- to 10-kHz bandpa] noise at 40 dB SPL, and in the presence of the same noise at 50 dB SPL. The noise was gated on at the beginning of each trial. Adult thresholds were the same in all three conditions, indicating that little or no sensory masking took place in the presence of the noise. Infant thresholds were about 10 dB higher in the presence of the noise. We term this effectdistraction masking. In Experiment 2, the effect of gating the noise on at trial onset was examined. Thresholds for the same tone were estimated in quiet and in the presence of the bandpass noise at 40 dB SPL, but the noise was presented continuously during the session. Under these conditions, distraction masking was still observed for infants. These findings suggest that a masker can have nonsensory effects on infants’ performance in a psychoacoustic task.

Using the Observer-Based Psychophysical Procedure to Assess Localization Acuity in Toddlers Who Use Bilateral Cochlear Implants

Hypothesis: Localization acuity will emerge in deaf children who receive bilateral cochlear implants (BI-CIs) before the age of 3 years but not in age-matched children who use a single device. Background: There is a growing clinical trend in which infants with severe-to-profound sensorineural hearing loss are receiving BI-CIs by 3 years. Although there is general agreement that better communicative and educational outcomes are achieved when the first implant is provided at a young age, there are few behavioral data showing the functional benefits of providing infants with BI-CIs. One potential benefit of BI-CIs is improved localization acuity, which develops within the first few years of life. Methods: Two groups of children with chronological ages ranging from 26 to 36 months participated: 1) children with normal hearing (n = 8) and 2) children with severe-to-profound sensorineural hearing loss (n = 18). Of the children who are deaf, 10 used BI-CIs, and 8 used unilateral cochlear implants. Localization acuity was measured with a single interval 2-alternative-forced choice right/left discrimination task, and minimum audible angles were computed at a performance level of 80% correct. Behavioral data were collected using the observer-based psychophysical procedure. Results: Preliminary results show that the observer-based psychophysical procedure is a feasible method to measure localization acuity in children with normal hearing and in deaf children with cochlear implants and that localization acuity is emerging in toddlers with BI-CIs but not yet in toddlers with unilateral cochlear implants. Conclusion: These data are among the first to show localization acuity in young children who use BI-CIs.

The relationship between auditory brainstem response and behavioral thresholds in normal hearing infants and adults

The nature of age-related improvements in auditory sensitivity was explored by comparing behavioral and auditory brainstem response (ABR) thresholds in 3- and 6-month-old infants and in adults. Thresholds were estimated for tone pips at 1, 4, and 8 kHz, presented at a rate of 13.3/s. The time course of development of the two response measures was compared, and the correlation between thresholds for individual subjects was examined. Infant ABR threshold was adultlike at all frequencies, even among 3-month-olds. Infant behavioral thresholds were elevated relative to adult thresholds. Between 3 and 6 months, significant improvement occurred in the 8-kHz behavioral threshold, but no improvement occurred at other frequencies. This difference between ABR and behavioral measures in developmental time course suggests that peripheral sensitivity is not a major determinant of behavioral threshold elevation during infancy. The correlation between behavioral and ABR thresholds was significant at 4 kHz for 3-month-olds and at 8 kHz for adults. This suggests that variability in sensory function at these frequencies contributes to both behavioral and ABR thresholds, although other factors are likely to be involved as well.

Human auditory development

The ability to detect, discriminate, and locate sounds improves throughout the course of infancy and childhood. This improvement stems from maturation of the conductive apparatus of the ear, of the primary auditory neural pathways, and of perceptual processes such as sound source segregation and selective attention.

Infant psychometric functions for detection: Mechanisms of immature sensitivity

Psychometric functions are described for individual 6- to 9-month-old infants and for individual adults for auditory detection of repeated, long- and short-duration tone bursts in quiet and for single, long-duration tone bursts in quiet and in noise. In general, infant psychometric functions have reduced upper asymptotes, shallower slopes, and poorer thresholds than adult psychometric functions. Infant-adult differences in slope and threshold are greater for short-duration tones than for other stimuli. Infant upper asymptotes are around 0.85 correct for all stimuli. One explantation for these findings is that infants are inattentive a certain proportion of time during the detection task. This model cannot account for the very shallow short-duration stimulus slope, nor can it account for infant-adult threshold differences for any stimulus. Other models of immature attention, or listening strategies, may be able to account for the slope and upper asymptote as well as the threshold of infant psychometric functions. Some combination of inattentiveness and primary neural immaturity may also account for the data. Although immaturities exist, some aspects of the detection process appear to be quantitatively similar in infants and adults.

Pure-tone sensitivity of 2- to 5-week-old infants

Abstract The behavioral response of 2- to 5-week-old infants to pure tones was examined. In the first experiment, tones at 500, 1000, and 4000 Hz at levels ranging from 25 tO 70 dB SPL were delivered to infants through a probe tube inserted in the ear canal. A trial consisted of five repetitions of a 500-ms tone burst at a fixed level, followed by 5 s of silence, and either five more repetitions of the same tone burst (signal trial), or 5 more s of silence (no-signal trial). An observer, blind to trial type, judged whether a signal or no-signal trial had occurred on the basis of the infants behavior. The observer also provided a confidence rating for the judgment on each trial. The results showed that at 500 and 4000 Hz, the observers performance improved as the level of the tone increased. Psychometric functions were fit to the data at these frequencies, combined over infants. The group thresholds calculated from these functions suggested similar sensitivity at 500 and 4000 Hz. There were some infants who contributed enough trials to establish an individual psychometric function. The average individual 4000-Hz threshold for these infants was lower than the estimate obtained from the group function, but sensitivity was still about the same at all three frequencies, relative to adults. Analysis of receiver operating characteristic curves generated from the confidence ratings, however, suggested that young infants are less sensitive to 4000-Hz than to 500-Hz tones when response bias is controlled. Finally, the results of a second experiment, in which the same procedure was used to examine the response of infants to 1000-Hz tone bursts at levels between 5 and 35 dB SPL, suggested that the infant threshold at this frequency is around 25 dB SPL. In the context at the first experiment, this further suggests that the range of sound intensities presented can dramatically influence the responses of infants to sound.

Vowel categorization by very young infants

Six-month-old infants are known to categorize vowels despite variation in talker voice and pitch contour. Using the observer-based psychoacoustic procedure, this study asked whether 2- and 3-month-olds could categorize similarly. Infants were trained to respond whenever the vowel category alternated from /a/ to /i/ and to refrain from responding when the vowel category remained the same, despite variation in spectral cues associated with pitch and talker changes. Eighty percent of 2-, 3-, and 6-month-olds did not respond the first time a talker change occurred in the absence of a vowel change, suggesting that even the younger infants recognize these spectrally different sounds as perceptually equivalent

Behavioral Studies of Hearing Development

This chapter reviews progress that points to the exciting potential in studies of behavioral development. The goal is twofold: to review common trends in psychoacoustic data from different species of newborn vertebrates and to indicate the many interesting questions that remain to be answered. The purpose of the introduction is to state and to justify the assumptions that underlie that goal.1

Observer-based Approaches to Human Infant Psychoacoustics

As subjects of psychoacoustical research, human infants, especially very young infants, pose special problems. Their response repertoire is limited; they make few unique responses to sound; and they are available for testing for only short periods of time. Our approach to the testing of infants has been to use existing spontaneous responses, but to use the infants’ behavior to judge whether or not a sound has occurred rather than whether or not a response has occurred. This observer-based approach has allowed us to assess sensitivity, frequency resolution, and temporal resolution in infants as young as 1 month of age.